1. Field of the Invention
The invention relates to a laser mask and a crystallization method using the same, and more particularly, to a laser mask whose shape is improved to reduce crystallization overlap regions such that the crystallization characteristics are improved.
2. Discussion of the Related Art
More attention has been recently paid to information display, and the desire to use portable information media increases. Attention has concentrated on the research and commercialization of a light and thin flat panel display (FPD) that replaces a cathode ray tube (CRT) used as a conventional display. In particular, among such FPDs, a liquid crystal display (LCD) device that displays images using the optical anisotropy of liquid crystals has high resolution, high quality of color display, and high picture quality such that LCD devices find applications as notebooks or desktop monitors.
An active matrix (AM) driving method is mainly used for liquid crystal display devices, where an amorphous silicon thin film transistor (TFT) is used as a switching element to drive the liquid crystal pixel portion.
An amorphous silicon TFT technology was conceptualized by LeComber et al. in 1979 and was commercialized as a three-inch liquid crystal portable television in 1986. Recently, a no less than 50-inch large TFT liquid crystal display device has been developed.
However, there is a limit to using an amorphous silicon TFT with an electrical mobility of <1 cm2/Vsec, which arises from a peripheral circuit that requires a high speed operation of no less than 1 MHz. Therefore, research has focused on simultaneous integration of a pixel portion with a driving circuit portion on a glass substrate using a polycrystalline silicon TFT (whose field effect mobility is larger than that of amorphous silicon TFT).
Polycrystalline silicon TFT technology has been applied to small modules such as camcorders since the liquid crystal color television was developed in 1982. A polycrystalline silicon TFT has low photosensitivity and high field effect mobility such that a driving circuit can be directly manufactured on a substrate.
Increasing mobility improves the operating frequency of the driving circuit that determines the number of driving pixels, such that it is possible to easily make a display having a fine pixel structure. Also, the time in which a signal voltage is charged to the pixel portion is reduced to thereby reduce the distortion of the transmitted signal such that improved picture quality can be expected.
Also, since the polycrystalline silicon TFT can be driven by a voltage of less than 10V, compared with the amorphous silicon TFT that has a high driving voltage of 25V, it becomes possible to reduce power consumption.
On the other hand, in order to manufacture the above-described polycrystalline silicon TFT, one uses a method of directly depositing a polycrystalline silicon thin film, a method of depositing an amorphous silicon thin film on a substrate, and performing thermal treatment on the amorphous silicon thin film to perform crystallization. In particular, a low temperature process is required in order to use an inexpensive glass substrate. In order to use an amorphous silicon TFT as an element of the driving circuit, it is necessary to increase the field effect mobility of the amorphous silicon TFT.
The current thermal treatment method for crystallizing the amorphous silicon thin film is divided into a solid phase crystallization (SPC) method and an eximer laser annealing (ELA) method.
According to the SPC method of forming a polycrystalline silicon thin film, after forming an amorphous silicon thin film on a glass substrate, the amorphous silicon thin film is heated at about 600° C. for several hours to dozens of hours to crystallize the amorphous silicon thin film. The polycrystalline silicon thin film obtained by the SPC method has relatively large grains, each having a size of several μm with many defects. The defects are known to have a bad effect on the performance of a TFT next to a gray boundary.
According to the eximer laser annealing method that is the basic method of manufacturing a polycrystalline silicon thin film at low temperature, a high-energy laser beam is instantaneously irradiated onto the amorphous silicon thin film for dozens of nsec to melt and crystallize the amorphous silicon thin film. Since amorphous silicon is melt and crystallized for a very short time period, the glass substrate is not damaged at all.
Also, the polycrystalline silicon thin film manufactured using eximer laser radiation has excellent electrical characteristics compared with the polycrystalline silicon thin film manufactured by the commonly used thermal treatment method. For example, the field effect mobility of the amorphous silicon TFT is about 0.1 to 0.2 cm2/Vsec, and the field effect mobility of the polycrystalline silicon TFT manufactured by a common thermal treatment method is about 10 to 20 cm2/Vsec. In comparison, the field effect mobility of the polycrystalline silicon TFT manufactured using the eximer laser exceeds 100 cm2/Vsec (IEEE Trans. Electron Devices, vol. 36, no. 12, p. 2868, 1989).
Hereinafter, a crystallization method using laser radiation will be described in detail.
FIG. 1 shows a graph illustrating the sizes of grains of a crystallized silicon thin film with respect to the density of radiated laser energy.
FIG. 1 has a first region I and a second region II in which the sizes of the grains of the crystallized polycrystalline silicon thin film increase as the laser energy density increases (IEEE Electron Dev. Lett., DEL-7,276,1986). However, in a third region III, when energy having no less than specific energy density Ec is radiated, the sizes of the grains of the crystalline polycrystalline silicon thin film reduce significantly.
That is, the crystallization mechanism of a silicon thin film varies with the density of the radiated laser energy, which will be described in detail.
FIGS. 2 to 4 depict sectional views illustrating a silicon crystallization mechanism in accordance with the laser energy density in the graph illustrated in FIG. 1, and which sequentially illustrate crystallization processes in accordance with the respective laser energy densities.
The crystallization mechanism of amorphous silicon by laser annealing is affected by various factors including laser radiation conditions such as laser energy density, radiation pressure, substrate temperature and the physical and geometric characteristics of the amorphous silicon thin film including absorption coefficient, heat conductivity, mass, impurity content, and thickness.
First, FIGS. 2A to 2C show that the first region I of the graph of FIG. 1 is a partial melting region, in which the crystallization of an amorphous silicon thin film 12 is performed only on the portion denoted by a dotted line. At this time, the size of a grain 30 of the grain structure is about several hundreds of Å.
That is, when the laser of the first region I radiates onto the amorphous silicon thin film 12 on a substrate 10 on which a buffer layer 11 is formed, the amorphous silicon thin film 12 melts. Strong laser energy radiates onto the surface of the amorphous silicon thin film 12 directly exposed to a laser beam, and relatively weak laser energy radiates onto the lower portion of the amorphous silicon thin film 12 such that only a predetermined portion of the amorphous silicon thin film 12 melts to perform partial crystallization.
In this laser crystallization crystal growing process, the surface of amorphous silicon first primarily melts by laser radiation. Second, latent heat generates by the solidification of the primary melting layer and accordingly a lower layer secondarily melts. Third, crystals grow by solidification. This crystal growing processes will be described in detail.
The temperature of the amorphous silicon thin film onto which laser light radiates is greater than the melting temperature of 1,000° C., such that the amorphous silicon film primarily melts to be liquid. Subsequently, the primary melting layer rapidly cools until a large difference is generated between the temperature of lower silicon and the temperature of the substrate, such that solid phase nucleation and solidification occur. The melting layer obtained laser radiation laser persists until solid phase nucleation and solidification occur. As long as ablation does not occur, the higher the laser energy density is or the smaller the amount of heat emitted to the outside is, the longer such a melting state is maintained. Also, since the primary melting layer melts at a temperature of 1,000° C., which is lower than the 1,400° C. melting temperature of crystalline silicon, the melting layer cools to be maintained in a super-cooled state where the temperature falls below the phase-transition temperature. The greater the super-cooled state, that is, the lower the melting temperature of the thin film (or faster the cooling speed is), the larger the solid phase nucleation rate becomes. Thus, it is possible to grow fine crystals.
When the primary melting layer is cooled to start solidification, crystals grow in an upward direction based on crystal nuclei. At this time, latent heat emits in accordance with the liquid phase to solid phase change of the primary melting layer, such that the lower amorphous silicon thin film secondarily melts and resolidifies. Repeating such processes grows the crystals. At this time, the lower secondary melting layer is more super-cooled than the primary melting layer such that the nucleus generating rate increases to reduce the size of the crystals.
Therefore, reducing the cooling speed during solidification improves the crystallization characteristics during crystallization by laser annealing. Accordingly, one may suppress the emission of the heat of the absorbed laser energy to the outside by heating the substrate, by irradiating a dual beam, and by inserting a buffer insulating layer to reduce the cooling speed.
FIGS. 3A to 3C show sectional views sequentially illustrating the silicon crystallization mechanism with respect to the second region II of the graph of FIG. 1. The second region II represents a near-complete melting region.
FIGS. 3A to 3C show a polycrystalline silicon thin film that has relatively large grains 30A, 30B, 30C of about 3,000 to 4,000 Å formed at the interface of the lower buffer layer 11. At this time, unmelted solid seeds 35 exist in the interface between the melting silicon thin film 12′ and the buffer layer 11, and the seeds operate as crystallization nuclei to produce horizontally grown crystals having relatively large grains 30A, 30B, 30C (J. Appl. Phys. 82, 4086).
However, the process window is very narrow since the crystallization can be performed only by irradiating laser energy that may remain on the interface between the melting silicon thin film 12′ and the buffer layer 11 onto the solid seeds 35 that are not to melt. Also, since the solid seeds 35 are non-uniform, the crystallized grains 30A to 30C of the polycrystalline silicon thin film have different crystallization directions, and hence, different crystallization characteristics.
Finally, FIGS. 4A to 4C show sectional views illustrating a crystallization mechanism with respect to a complete melting region represented by the third region III of the graph of FIG. 1.
As illustrated in FIGS. 4A to 4C, the energy density corresponding to the third region III results in the formation of very small grains 30.
That is, when the laser energy density is no less than a predetermined level Ec, sufficient energy is applied to the amorphous silicon thin film 12 such that the amorphous silicon thin film 12 completely melts. As a result, solid seeds that can grow to become grains are not present. Then, the silicon thin film 12′ onto which strong laser energy is irradiated melts the silicon thin film 12′ such that it undergoes a rapid cooling process in which uniform nuclei 30 are generated. As a result, fine grains 30 are formed.
On the other hand, in order perform the laser crystallization, an eximer laser annealing method using a pulse-shaped laser finds predominant use. However, a sequential lateral solidification (SLS) method in which grains are laterally grown to significantly improve the crystallization characteristics has recently been suggested and is under active study.
The observation that grains are vertically grown with respect to a boundary between liquid phase silicon and solid phase silicon on the boundary (Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452,956-957, 1997) is used as a basis for an SLS method. According to this SLS method, the magnitude of laser energy and a range in which a laser beam is irradiated are properly controlled to laterally grow grains having a predetermined length such that the size of the silicon grains can be increased.
Such an SLS method will be described in detail with reference to FIGS. 5A to 5C, which are sectional views sequentially illustrating crystallization processes in accordance with the SLS method.
FIG. 5A shows that when a laser having no less than the energy density (that is, the above described third region III of FIG. 1) in which the amorphous silicon thin film 112 is irradiated such that it completely melts.
At this time, the region onto which laser light irradiates and the region onto which laser light is not radiated are formed via a patterned mask.
FIGS. 5B and 5C show that since laser of sufficient energy is irradiated onto the amorphous silicon thin film 112, the amorphous silicon thin film 112 completely melts. However, since the amorphous silicon thin film 112 melts using beams spaced at uniform intervals, crystals are grown using solid phase silicon that exists at the boundaries between the silicon thin film 112 onto which laser light is not irradiated, and the melting silicon thin film 112′ has nuclei.
That is, immediately after the radiation of laser energy is complete, the melted silicon thin film 112′ is cooled starting at the right and left surfaces, which are regions onto which laser is not irradiated. This occurs because the solid phase silicon thin film 112 has larger heat conductivity on the right and left surfaces than the heat conductivity of the buffer layer 111 or the glass substrate 110 under the silicon thin films 112 and 112′.
Therefore, in the silicon thin film melt 112′, the right and left solid phase and liquid phase interfaces first reach the nucleus forming temperature before the central portion, such that the crystal nuclei are formed in the right and left portions. After the crystal nuclei are formed, grains 130A and 130B grow laterally from the sides that have lower temperature toward the region with higher temperature. That is, crystals grow from the interfaces to the central portion.
Larger grains 130A and 130B are formed due to this lateral growth of crystals, and processes using the energy of the third region III create a wide process window.
On the other hand, a slit shaped mask that has a region that transmits light and a region that blocks light is used for the laser crystallization, which will be described in detail below.
FIG. 6A depicts a plan view illustrating an example of a conventional laser mask. FIG. 6A shows that the laser mask 270 has a slit pattern 275 that contains rectangular transmitting regions 273 with predetermined width and length.
That is, the laser mask 270 has rectangular transmitting regions 273 that transmit light and a blocking region 274 that blocks light. The laser beam that transmits through the transmitting regions 273 of the slit pattern 275 crystallizes a predetermined silicon thin film area in the shape (that is, a rectangle) of the slit pattern 275.
However, as illustrated in FIG. 6B, the edge E of the crystallized silicon thin film does not actually crystallize in the shape of a mask pattern. That is, the slit pattern 275 becomes rounded due to the diffraction of the laser, which will be described in detail below.
In FIG. 6B, the dotted line at the edge E of the crystallized silicon thin film denotes the shape of the slit pattern 275 of the mask 270 used for the crystallization.
FIG. 7 shows a plan view illustrating an enlargement of the edge E of the crystallized silicon thin film illustrated in FIG. 6B. FIG. 7 shows that since complete melting energy that entirely melts the silicon thin film is irradiated onto the central region A of the edge E, a crystallized pattern is formed having the same shape as the slit pattern 275. However, the laser beam diffracts at the edge of the slit pattern 275 corresponding to the corner B of the edge E, such that partial melting energy (that does not completely melt the silicon thin film) is irradiated onto the corner of the edge. As a result, the edge E of the crystallized thin film becomes convex.
This occurs because the edge E of the crystallized silicon thin film grows by using the amorphous silicon thin film positioned on the boundary (and that melts to be round) as a nucleus, and a second grain 230B that grows in a different direction from the direction of the first grain 230A is consequently formed. That is, the second grain 230B has different crystallization characteristics from those of the first grain 230A. As a result, discontinuous regions with different crystallization characteristics are found in the crystallized silicon thin film.
At this time, the discontinuous region forming the convex edge E of the crystallized silicon thin film has different crystallization characteristics along the width W thereof, and in order to apply the silicon thin film to a liquid crystal display device, it is necessary to reduce the width of the discontinuous region.
Alternately, common processes of crystallizing a silicon thin film using the above-described mask will be described below.
FIGS. 8A to 8C show plan views sequentially illustrating processes of crystallizing the silicon thin film using the related art mask illustrated in FIG. 6A.
FIG. 8A shows the mask 270 illustrated in FIG. 6A being positioned on a substrate 210. A first laser beam is irradiated onto the mask 270 such that an amorphous silicon thin film 212 deposited on the substrate 210 crystallizes.
At this time, the crystallized regions correspond to the transmitting regions 273 of the mask 270. Two crystallized regions with predetermined length will form if it is assumed that two transmitting regions of the mask 270 are present.
That is, when the first laser beam irradiates onto the surface of the substrate 210 through the conventional mask 270 that has two rectangular slit patterns 275, the silicon thin film onto which the laser is radiated through the slit pattern 275 has a first grain 230A. This first grain 230A grows laterally with the amorphous silicon thin film 212 positioned on the up and down boundaries as a nucleus.
At this time, the edge of the crystallized silicon thin film 212′ does not actually crystallize in the shape of the mask pattern. That is, the slit pattern 275 is rounded due to the diffraction of the laser beams described above. At the edge of the round crystallized silicon thin film 212′, a second grain 230B grows using the amorphous silicon thin film 212 positioned on the round melting boundary as a nucleus. The second grain 230B therefore forms in a different direction from that of the first grain 230A.
That is, the second grain 230B has crystallization characteristics different from those of the first grain 230A, and a discontinuous region exists in the crystallized silicon thin film.
When the first crystallization is completed, a stage (not shown) on which the substrate 210 is placed (or alternately the mask 270) is moved less than the length of the mask pattern (that is, the slit pattern 275). Then, a second laser beam irradiates such that crystallization is continuously performed in the X-axis direction.
That is, for example, when the stage moves in the X-axis direction such that the slit pattern 275 overlaps the discontinuous regions 280 of the crystallized silicon thin film 212′, and then the second laser beam irradiates the surface of the substrate 210, as illustrated in FIG. 8B, then second crystallized patterns 212″ (which are identical to the silicon thin film patterns 212′) laterally crystallize to overlap the discontinuous regions 280 of the primary crystallized silicon thin film 212′.
Subsequently, a third laser beam is irradiated onto the surface of the substrate 210 by the same method, and third crystallized patterns 212′″, identical to the second crystallized silicon thin film patterns 212″, overlap the discontinuous regions 280 of the second crystallized silicon thin films 212″.
Consequently, the larger the width W of each of the discontinuous regions 280 becomes, the larger the overlap region of laser beams for the next shot results. As a result, the crystallization time increases. That is, the discontinuous regions 280 of the crystallized silicon thin films 212′, 212″, and 212′″ have different crystallization characteristics such that the silicon thin film around the discontinuous regions 280 fails to crystallize and remains as amorphous silicon 212. Therefore, the next shot must be irradiated such that the discontinuous regions 280 overlap with each other.
After crystallization in the X-axis direction completes by the above method, the mask 270 (or the stage) moves in the Y-axis direction (in the —Y-axis direction when the stage is moved) by a predetermined distance.
As illustrated in FIG. 8C, a laser irradiation process is repeated on the portion on which the primary crystallizing process was performed in the horizontal direction.
When the crystallization is performed using the related art mask, the polycrystalline silicon thin film includes multiple first regions P1 that have normal grains, and multiple second regions P2 that exist between the first regions P1, and these second regions P2 are discontinuous regions with different crystallization characteristics.
The discontinuous regions with different crystallization characteristics make the liquid crystal display device manufactured from the crystallized thin film have non-uniform characteristics such that the quality of the liquid crystal display device deteriorates.
Also, when the crystallization is performed using the laser mask, the crystallized silicon thin film has edges at which discontinuous regions with different crystallization characteristics are formed due to the diffraction of laser beams. Therefore, the silicon thin film around the discontinuous regions does not crystallize and remains as amorphous silicon. Therefore, the next shot must be irradiated such that the discontinuous regions overlap. As a result, the larger the width of each of the discontinuous regions requires an increase in the overlap region of the laser beams for the next shot. Accordingly, the crystallization time increases.